The invention relates to a method and an apparatus for charging a nickel-cadmium battery. More particularly, the invention permits a nickel-cadmium battery to receive a full charge without a large rise in the temperature of the battery.
Rechargeable nickel-cadmium batteries are presently used in a number of industrial and consumer applications, for example, in communications (such as FM transceivers), medical, photographic, and battery analysis equipment. When recharged, a high battery charging rate is desirable in order to reduce the charging time and, therefore, the time a battery is out of service. Nickel-cadmium (Ni-Cd) batteries are known which are able to withstand a relatively high charging rate. In a conventional fast charge, a current of amperage that is numerically equal to the battery capacity C, in ampere-hours, is supplied to the battery for approximately one hour. This rate is commonly referred to as the C/1 rate. As the battery reaches full charge, the charging rate is reduced or charging is stopped. It is important to control the cutoff of charging so as to assure, on the one hand, that the battery has been fully charged, but to prevent, on the other hand, over-charging which may damage the battery cells.
A known method for battery charging is to apply a fast charge while monitoring the increase in battery voltage, until a predetermined battery voltage is reached. Voltage monitoring methods are unreliable inasmuch as the battery voltage during charging depends upon the condition of the battery prior to instituting the charge and the ambient conditions during the charge, not just the time from the beginning of the charge. Therefore, by controlling the charging cutoff responsive to voltage, a battery can be over-charged or may not be fully charged upon termination of the high charge rate.
Eveready has proposed, in an article entitled "`Eveready` Fast Charge of Nickel-Cadmium Cells" that high-rate charging of fast-charge Ni-Cd batteries be controlled by monitoring battery temperature. As noted therein, at the C/1 rate, the battery temperature increases rapidly as the battery approaches full charge, since the electrical energy supplied to the battery is not converted and stored chemically in the battery but changed into heat.
This rapid temperature increase, indicative of full charge, is used to control the cut-off of a high charging rate, or fast charge, applied to a fast-charge Ni-Cd battery. Several temperature monitoring methods are described. In one temperature monitoring method, the fast charge is stopped when a predetermined battery temperature is exceeded. In another temperature monitoring method, the fast charge is stopped when the rate of change of the battery temperature exceeds a specified value. Both temperature monitoring methods suffer from the need to employ a temperature sensor in close proximity to the battery, which necessitates additional terminals and connections, and additional expense. Moreover, the temperature monitoring methods may be inaccurate because the battery temperature will be a function of and will change with the ambient temperature. Furthermore, temperature sensors generally respond more slowly than sensors for electrical parameters.